Fault detection, e.g. locating faults such as breaks, shorts, discontinuities, degraded components, and improperly terminated transmission lines, is a test performed by CATV service providers in order to pinpoint problems in the cable distribution network. Faults within the distribution network are typically characterized by an impedance mismatch, i.e. the impedance of the fault is different than the characteristic impedance of the transmission lines of the distribution network. For example, transmission lines in a CATV distribution subsystem typically have an impedance of approximately 75 ohms; however, a short on the transmission line would have an approximately zero impedance and a break would have an approximately infinite impedance.
One problem with faults in the distribution subsystem is that faults, due to their impedance mismatch characteristics, reflect signals transmitted through the distribution network. As a result, faults in the distribution network may also cause problems throughout the distribution network due to interference from reflected signals. Therefore, it is important for CATV service providers to be able to easily identify and locate faults within the network in order to cure reception problems of a single subscriber and to remove fault generated interference from the distribution network as a whole.
Frequency domain reflectometry utilizes a reflectometer that applies a sweep signal to a distributed communication network. The sweep signal is an RF signal that is swept from an initial frequency to a final frequency, e.g. 5 MHz to 82 MHz, in relatively small increments, e.g. 0.075 MHz. If an impedance mismatch exists within the network the impedance mismatch will reflect each transmitted signal back to the reflectometer at the same frequency as the transmitted signal, but retarded in phase. As a result of this reflection, a standing wave is generated. The reflectometer measures the level of the standing wave at each swept frequency in order to obtain a reflected sweep response signal. The retardation of the reflected sweep response signal is such that the minimums of the reflected wave will align to ½ the wavelength of the impedance mismatch from the reflectometer. Due to this known relationship, the reflectometer may determine the distance from the reflectometer to the impedance mismatch.
Frequency domain reflectometry (FDR) systems have been used to test networks, such as the one disclosed in U.S. Pat. No. 5,994,905, issued Nov. 30, 1999 to Franchville; U.S. Pat. No. 6,177,801, issued Jan. 23, 2001 to Chong; U.S. Pat. No. 6,466,649, issued Oct. 15, 2002 to Walance et al; U.S. Pat. No. 6,959,037, issued Oct. 25, 2005 to Bailey et al; and U.S. Pat. No. 7,071,700, issued Jul. 4, 2006 to Gorka et al.
Unfortunately, the results of previous FDR systems are typically displayed as a simple graph with distance on the X-axis and reflection amplitude on the Y-axis. The graphical results include several false readings, e.g. harmonics and erroneous reflections, and require a great deal of interpretation by a technician. Filtering processes have been utilized to cut down on the anomalies, but the results are still prone to interpretation errors and there is no definitive means of determining what kind of device is causing each reflection.
An object of the present invention is to overcome the shortcomings of the prior art by providing a system that utilizes the raw data to identify devices in a cable network, such as splitters, bad barrels as well as cables that are open or shorted, and that displays the results in a tabular format with a description of the device type and their distance from the test location.